關於phy力學問題normal reaction force

Take downward quantities to be positive.

There are 3 forces needed to be consider:
(1) gravitational force acting on the body, F1 = mg
(2) the body exerting its weight (F2 = -mg) on the ground, F2 = F1 = mg
(3) normal reaction force acting to the body by the ground, F3 = -F2 = -mg

F1 and F3 exert on the body, but not F2.
Resultant force = F1 + F2 = mg + (-mg) = 0
It reaches equilibrium as the resultant force is equal to zero.